The invention disclosed herein relates to a method for monitoring the performance of a switching system such as a switch mode converter, and the circuitry for carrying out the method. The invention relates more specifically to a method and circuitry for detecting an abnormal condition in a switch mode converter.
The term "switch mode converter" is used herein in a broad sense and, unless otherwise indicated, encompasses DC to DC, AC to DC and AC to AC switch mode converters. The term "switch" is also used herein in a broad sense and can encompass one or more individual switch elements.
In the specific case of DC output voltage switch mode converters, they convert a relatively or completely unregulated input voltage, for example, a raw DC voltage, to a regulated DC output voltage, and typically include a primary switch means which chops the unregulated input voltage and converts it to an AC voltage, and a rectifier and filter circuit which rectifies the AC voltage coupled thereto, e.g., by a transformer, and filters the rectified AC voltage to provide the DC output voltage. Typically, the unregulated input voltage is the primary source of power for the converter, or it is obtained from the converter's primary source of power, e.g. the input voltage is a raw DC voltage produced by rectifying and peak charging an AC voltage obtained from the primary AC line. The DC output voltage of the converter is regulated by applying control signals to the primary switch means to vary its duty cycle and, hence, the duty cycle of the AC voltage obtained from chopping. In many converters, a signal, which may be used for warning and/or control purposes, is generated when an abnormal condition such as a loss of primary power or input voltage to the converter is detected. This abnormal-condition-indicating signal, sometimes referred to as a "power fail detection" or "PFD" signal in the case of primary power and input voltage failure-related conditions, should be generated before the converter reaches a point where any of its output voltages fall below the lower limit of an acceptable range. Detecting a loss of primary power or input voltage in switch mode converters is commonly accomplished by power fail detection circuits which monitor the voltage of the primary source of power to the converter or a voltage obtained from the primary source of power, and compare the monitored voltage with a power fail threshold voltage. When the monitored voltage falls below the power fail threshold voltage, a PFD signal is generated and a power fail condition is detected.
The general technique described above for detecting a power fail condition in a switch mode converter is not entirely satisfactory. Although there is a relationship between the unregulated input voltage to the switch mode converter and the ability of the converter to function properly, that relationship is very complex and is not linear. Therefore, an adjustable component is typically provided in the power fail detection circuit or the converter, and an adjustment is made during testing to cause the power fail detection circuit to generate a PFD signal at a given power fail threshold voltage. However, in order to ensure proper operation of the converter throughout a specified range of operating parameters, the adjustment is made to cause the power fail detection circuit to generate a PFD signal for worst case load conditions. As a result of this worst case adjustment, a converter operating at less than worst case conditions could cause a PFD signal to be generated before proper operating performance of the converter is actually threatened. A direct consequence of such premature power fail detection is that the converter operates in a voltage range of the primary power source which is less than it could otherwise be, which is a significant drawback for converters.
There is thus a need for a method and circuitry for reliably and accurately detecting an actual or imminent abnormal condition such as a power fail condition in a switching system such as a switch mode converter while avoiding false or premature detection of an abnormal condition when, in fact, none exists or is imminent.